I have a Sears 20 gauge semi-auto with an integral choke which can be turned to full or modified. The model is 75 (583 7500). The gun weight is 7.6 lbs empty. On the fore stock is a lever that shifts between heavy and light loads. I will use the gun for small game and am limited to shooting loads that generate no more than 20 ft/lbs. Can you tell me how to calculate ft/lbs for any given load? Alternatively, can you recommend specific shot weights/velocities that will keep me below the 20 ft/lbs of recoil?

But just remember that all recoil isn’t the same even though the numbers might be the same. 20 ft/lb from a fixed breech gun like an O/U, SxS or pump, will be delivered in a very short period of time. It will feel like a poke. The same 20 ft/lb delivered from a gas operated semi-auto will be received over a slightly longer period of time due to the action of the gun and will feel more like a push. Poke vs push. They certainly will feel different, but the measurement of calculated recoil is the same.

And then there is gun fit. If the gun’s firing makes the stock hit your face, it will feel like excessive recoil, but it is really due to gun fit. If it hits you in the shoulder and that’s where you feel it, then it’s pure recoil.